The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a method and apparatus for tracking motion, such as anatomical movement, between MR images for efficient and effective MR image registration in blood oxygenation level dependent magnetic resonance imaging, and particularly for indexing myocardial perfusion reserve.
The ability to track motion in a time series of images is essential to a number of MRI applications, and particularly to applications involving imaging of, or within, moving organs. In these applications, it is important to be able to track motion not only to appropriately and accurately locate a region of interest, but also to apply motion artifact correction techniques. Motion artifact correction techniques are important in a number of applications including MR angiography (MRA) of coronary arteries; functional MR imaging (MRI) of brain physiology; and heart function monitoring to assess the severity and extent of damage in ischemic heart disease. Another important application in which motion tracking is required for proper MRI analysis is in myocardial perfusion imaging, which can be used to estimate the myocardial perfusion reserve, or blood flow through the heart.
Myocardial perfusion imaging is typically performed by injecting a contrast agent, such as gadolinium (or Gd-TPA), and obtaining a time series of data indicative of the kinetics of the contrast agent as it moves through the heart. While this process has proved successful in providing high signal to noise ratio (SNR) images for characterizing perfusion and perfusion reserve, there are certain disadvantages associated with contrast agent methods. Particularly, due to the need to inject an exogenous contrast agent, it is not possible to perform repeated studies within a short period of time using this method. Contrast agent methods, therefore, generally cannot be used to efficiently provide serial studies of heart conditions.
A promising alternative method of myocardial perfusion imaging is blood oxygenation level dependent (BOLD) MRI. In BOLD MRI, sensitivity to deoxyhemoglobin is used as a natural paramagnetic contrast agent to determine blood flow changes in the body, and exogenous contrast agents are therefore not required. Transverse relaxation-time(T2*) images, which have a signal intensity that is inversely related to deoxyhemoglobin concentration are acquired and analyzed for a patient in both a stressed and an unstressed state to determine changes in signal intensity. The changes in signal intensity correlate to changes in oxygenation level, and therefore provide a means for indexing the blood flow changes or myocardial perfusion reserve.
While BOLD MRI offers a number of advantages over previous methods of imaging myocardial perfusion by, for example, eliminating the need for an exogenous contrast agent and making repeated studies possible within relatively short time period, prior art methods of using BOLD MRI for myocardial perfusion analysis have proved to be relatively inaccurate. Critical inaccuracies relate to the inability to adequately register low SNR relaxation time images of moving bodies. Known pattern matching techniques such as least squares and cross-correlation techniques, have proven less than entirely effective when applied to these very low SNR relaxation time weighted images and particularly when applied to T2* images of moving bodies. When using BOLD MRI, therefore, a large portion or even all of the images are generally registered manually. This process is extremely time consuming and prone to operator error, thereby making BOLD MRI analysis impractical.
There remains a need, therefore, for a method and apparatus that can accurately and quickly register time series of relaxation-time images to track motion, and particularly for such a method and apparatus which is suited for use in myocardial perfusion imaging.
The present invention is a method and apparatus for using magnetic resonance imaging (MRI) techniques to register and analyze time series of relaxation-time images, and particularly for analyzing images of a moving body characterized by a significant degree of noise. Generally, in the method of the present invention, an anatomical image having a high contrast between the background and the region of interest, such as the myocardial region, is reconstructed in conjunction with each of a time series of relaxation-time images indicative of flow or motion of a body. Successive high contrast anatomical images are registered, or mathematically aligned, to accurately track motion of the body between successive images. The registration data can then be applied to the lower SNR relaxation-time images, and used to identify regions of interest for analysis, allowing for semi-automatic registration of the relaxation-time images.
In a preferred embodiment, the method of the present invention is used to derive a parameter characterizing the ability to augment tissue blood flow or oxygenation in response to stress using BOLD MRI, wherein the characterization preferably takes the form of an xe2x80x9cindexxe2x80x9d illustrating the relative change in blood flow between a rest and a stressed state. The derived parameters can be used to index myocardial perfusion reserve, a measure of the ability of the heart to meet increased metabolic demand under stress. Here, a repeated series of transverse relaxation-time (preferably T2*) images are taken for each of a rest and a stressed state, thereby defining a time-series. Such images provide an indication of oxygenation levels in the heart, and can therefore be used to determine blood flow changes in the heart. However, due to signal intensity decay with time, transverse relaxation-time images generally have a low SNR. To locate the desired regions of interest in the collected T2* images, a second set of higher SNR images are acquired or constructed. These can be, for example, proton density images mathematically extrapolated for an echo time substantially equal to zero, providing a high contrast image wherein the background is substantially dark or black and the region of interest is substantially light or white. The high contrast images provide a relatively high definition, stable anatomical image of the body of interest for comparison purposes. These images can be analyzed efficiently with a number of known pattern matching techniques, and can be used to track motion of a region of interest at successive times. The known change in position can then be applied to the low SNR images to determine the position of the region of interest for analysis of the relaxation-time images, thereby providing a fast, semi-automatic analysis of the images which compensates for potential misregistration due to breathing and cardiac motion by utilizing higher contrast images in obtaining the registration data.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.